The present invention relates to a radio transmitter having particular, but not exclusive, application in digital communication systems such as GSM.
The GSM specification for spurious emissions from a mobile station transmitter, as defined in the European Telecommunication Standards Institute (ETSI) document xe2x80x9cGSM: Digital Cellular Telecommunications System (Phase 2) Radio Transmission and Receptions (GSM 05.05 version 4.19.0)xe2x80x9d, is summarised in FIG. 1 of the accompanying drawings. The figure plots the permitted levels of unwanted noise N in a 1 Hz bandwidth, referenced to a carrier power level of +33 dBm at 902 MHz, against frequency f in MHz. Also shown are the positions of the GSM transmit (Tx) and receive (Rx) bands. In the portion of the GSM receive band between 925 and 935 MHz the noise must be held below xe2x88x92150 dBc, and in the portion between 935 and 960 MHz the noise must be held below xe2x88x92162 dBc. Such low levels of noise are difficult to achieve with a fully-integrated transmitter, and to meet this specification with a conventional architecture it is necessary to use an expensive RF filter after the final stage of power amplification, with a consequent loss of transmitter efficiency.
A block diagram of a conventional transmitter architecture, which performs dual up-conversion in analogue circuitry, is shown in FIG. 2 of the accompanying drawings. Digital data for transmission is provided as an input 202 to a Gaussian Minimum Shift Keying (GMSK) modulator 204 which produces as output analogue in-phase I and quadrature phase Q signals on a zero-frequency carrier. The I signal is supplied to a first IF mixer 206, and the Q signal is supplied to a second IF mixer 208. An output signal from a first Voltage Controlled Oscillator (VCO) 210 is supplied via a first 90xc2x0 phase shifter 212 to the local oscillator port of the first IF mixer 206, and directly to the local oscillator port of the second IF mixer 208. The resultant output signals from the mixers 206, 208 are added together by a combiner 214 and filtered in a bandpass filter 222 to produce a signal at the required IF frequency, for example 100 MHz. As well as removing unwanted mixing products the bandpass IF filter 222 reduces levels of out-of-band noise. The filter 222 is commonly implemented off-chip.
The first VCO 210 is driven by a signal produced by an IF synthesiser 216 which derives its output using a 13 MHz reference oscillator 218 under the control of instructions passed on a control bus 220 to produce a fixed IF output.
The filtered IF signal is split into two parts. The first part has its phase shifted 90xc2x0 by a second phase shifter 224 and is then up-converted by a first RF mixer 226, the second part of the IF signal is up-converted by a second RF mixer 228. An output signal from a second VCO 230 is supplied directly to the local oscillator port of the second RF mixer 228, and via a third 90xc2x0 phase shifter 232 to the local oscillator port of the first RF mixer 226. The resultant output signals from the mixers 226, 228 are added together by a combiner 234 to produce a combined RF signal including a product at the required frequency in the GSM transmit band between 880 and 915 MHz.
The second VCO 230 is driven by a signal produced by an RF synthesiser 236 which derives its output using a 13 MHz reference oscillator 218 under the control of instructions passed on a control bus 220 to produce a variable output frequency.
Without extra filtering, noise from the second VCO 230 would fall into the GSM receive band at an unacceptably high level. The RF signal therefore passes through a first RF bandpass filter 238 before being amplified for transmission by a power amplifier 240. The amplifier 240 is normally operated under heavy compression for best efficiency, and this has the effect of removing the AM component of single-sideband noise on the input signal. Without the AM component, the residual FM component produces noise at equal levels on the two sides of the signal, largely restoring noise in the unwanted sideband. Hence the signal must be filtered by a second RF bandpass filter 242 before transmission via an antenna 244. The second RF filter 242 is much less desirable than the first filter 238 both in terms of cost (because of the higher power levels it must handle) and because of the resultant loss in transmitter power due to losses in the filter 242. These transmitter losses can amount to more than 1 W, requiring the use of a bigger power amplifier 240 and a larger battery.
Such an architecture therefore has a number of disadvantages for use with current digital cellular communications standards. It is difficult to use for telephones operating in accordance with two or more standards unless the schemes are compatible (in the sense of having similar requirements for bandwidth and modulation schemes, for example). This is because only the baseband circuitry is digital, and the analogue IF and RF circuitry is inherently less flexible. Also, as mentioned above, it is difficult to meet the GSM requirements for spurious emissions without additional filtering after the power amplification stage.
An object of the present invention is to address the problems described above.
According to the present invention there is provided a radio transmitter comprising modulation means for producing quadrature modulated signals, first frequency-translation means for translating said signals to a variable intermediate frequency (IF) signal in digital form, digital to analogue conversion means for converting said variable IF signal to analogue form, second frequency-translation means for translating said analogue IF signal by a fixed frequency to a radio frequency (RF) signal, and power amplifying means for amplifying said RF signal for transmission.
The present invention is based upon the recognition, not present in the prior art, that digital up-conversion to a variable IF provides a more flexible transmitter architecture that does not require expensive RF filtering.
An advantage of the described transmitter architecture is that it is extremely versatile, giving the possibility of changing modulation methods, frequencies, sampling rates or bandwidths to accommodate a variety of communication standards.
Advantageously, error correction means are provided between the first and second frequency translation means, to correct for the imbalance between in-phase and quadrature signals in the second frequency translation means.
Provision of such means enables automatic calibration of the transmitter during manufacture to take account of the imbalance between signal paths in the second frequency-translation means, after which calibration no further attention is required.